1. Field of the Invention
The present invention relates to an AC adapter for a portable device.
2. Description of the Related Art
FIG. 1 is a perspective view showing a conventional AC adapter 1 with an AC cord. The AC adapter 1 includes a switching regulator circuit embedded in a synthetic resin case 4 prepared by combining a lower half case 2 with an upper half case 3. An AC cord 11 having an AC plug 10 at a tip thereof extends from one side of the case 4 and a DC cord 21 having a plug 20 at a tip thereof extends from the other side of the case 4. And a name plate 22 is attached to a surface of the case 4.
The name plate 22 includes a synthetic resin sheet in a front surface thereof and an aluminum sheet portion 23 for diffuse radiation in the whole area of a back surface thereof. The name plate 22 has substantially the same size as that of the surface of the case 4. The aluminum sheet portion 23 covers substantially the whole area of the surface of the case 4.
The AC adapter 1 is used by inserting the AC plug 10 into an outlet and connecting the plug 20 to a portable device. A noise is generated in each on/off operation repeatedly performed by a switching regulator circuit. Such a noise may be partially transmitted to the AC plug 10 via the AC cord 11. The noise transmitted via the AC cord 11 is referred to as “noise terminal voltage”. This noise terminal voltage is transmitted to other electronic devices via the outlet and home wiring and affects the electronic devices connected to other outlets. Thus, it is preferable to reduce the noise terminal voltage in the AC adapter 1 as much as possible.
In the conventional AC adapter 1, the aluminum sheet portion 23 covers substantially the whole area of the surface of the case 4 and a portion of the aluminum sheet portion 23 reaches a portion for an AC input terminal to which the AC cord 11 is connected.
In FIG. 2, line I indicates the result of measurement of noise terminal voltage in the conventional AC adapter 1 using a spectrum analyzer and a receiver. The horizontal axis indicates frequency and the vertical axis indicates noise levels. As the test conditions, Vin is 120V/60 Hz and Io is 2A. In FIG. 2, line II indicates an average noise terminal voltage (AV) determined by the standards and line III indicates a quasi peak value (QP) of a peak noise terminal voltage.
As shown in FIG. 2, although the noise terminal voltage meets the standards, a margin thereof, such as a margin M relative to the peak noise terminal voltage is relatively small.
Accordingly, in the AC adapter, it is required to enlarge the aforementioned margin by further reducing the noise terminal voltage in order to improve the quality of the AC adapter.